1. FIELD OF THE INVENTION
The present invention relates to an ink-jet printing apparatus and an ink-jet printing method, and more specifically to reduction of an ink mist generated in association with ink ejection.
2. DESCRIPTION OF THE PRIOR ART
Recently, color printing, high-speed printing and high-quality printing in ink-jet printing methods have been remarkably improved. Also, the spread of Internet communications, digital cameras and the like has increased a demand for photographic quality color printing.
As methods capable of achieving such high-resolution, high-gradation, and high-quality printing in an ink-jet printing method, following are known:
(1) A method in which a size of a nozzle for ejecting ink is reduced to increase a nozzle arrangement density in a printing head, and a size of an ink droplet itself ejected from the nozzle is decreased to print a small-sized dot, thereby improving the resolution of a printed image. PA0 (2) A method in which, without increasing the nozzle arrangement density, a plurality (at least two) of heads including a head for ejecting a high-concentration ink and a head for ejecting a light ink lower in concentration than the above high-density ink are prepared for one color. Overlaying one of these inks with another is performed as necessary, thereby improving gradation of a printed image to increase the quality of the image. PA0 (3) A method in which, without increasing the nozzle arrangement density, an amount of ink ejected from nozzles is made variable to change a size of a printed dot in a relatively large range, thereby improving gradation to increase image quality.
However, it is said that the above method (3) is generally difficult to be executed in a so-called bubble-jet system (hereinafter also referred to as BUBBLE-JET system) in which a bubble is formed in ink by means of a thermal energy generation element and the ink is ejected by the pressure of the bubble generation.
On the other hand, the above method (2) causes a problem such that the number of printing heads used is increased so that a cost of an apparatus may be increased in the case that a number of types of concentrations for a single color is increased to more than two types. For this reason, it is common to use two types of inks of dark and light for a single color. In this case, since a number of density gradation levels is restricted to a small number, there is a limitation in pursuing high image quality.
In view of the above discussion, a method such as the above-stated method (1), in which printing is performed by ejecting a relatively small ink droplet from a single nozzle, has an advantage in improving image quality in the BUBBLE-JET system. As a method for ejecting such a small liquid droplet (less than 10 pl in ejection volume), a method belonging to a kind of the BUBBLE-JET system is known in which a generated bubble is communicated with an outer atmosphere in a vicinity of an ejection port before an ink droplet separates from the ejection port (hereinafter this method is referred to as "bubble-through system" or "BTHJ system"). For example, this type of method is described in Japanese Patent Application Laid-open No. 10940/1992, Japanese Patent Application Laid-open No. 10941/1992, Japanese Patent Application Laid-open No. 10742/1992 or the like. It should be noted that, hereinafter, only a method of the BUBBLE-JET system except the BTHJ system, that is, a method for performing ejection without communicating with ambient air in the vicinity of the ejection port, is referred to as the BUBBLE-JET system.
In the BUBBLE-JET system, in order to make the ejected ink droplet as small as possible, it is necessary to make thinner a liquid passage communicating with the ejection port from which the ink droplet is ejected. In this case, ejection efficiency is deteriorated or an ejection speed is decreased. When the ejection speed is decreased as above, an ejection direction or an ejection amount becomes unstable. Furthermore, this decreasing of the ejection speed easily causes an unstable ejection, an initial ejection failure, or the like, which are caused by an effect of viscosity increase due to evaporation of ink moisture during a non-ejection state of the printing head. This results in a reliability problem.
On the other hand, the BTHJ system is suitable for ejecting the small liquid droplets. Furthermore, since mainly a geometrical shape of the nozzle determines the amount of ejected liquid droplets or the like, the BTHJ system has an advantage that it is not easily affected by temperature or the like and the ejection amount of the liquid droplets is relatively stable as compared with the BUBBLE-JET system.
However, in general, as the size of liquid droplets ejected from the nozzle decreases, the number of ink application times required for covering a predetermined printing area increases. As a result, an amount of ink satellites generated in association with ejection of an ink droplet increases with an increase in the number of application times. Further, the increase in the number of ink application times may result in a reduction of a throughput of an apparatus. In order to make up for the reduction, an increase in a number of the nozzles may be considered. However, this further increases generation of the satellites.
As described above, when the amount of the satellites increases, a problem occurs in that a mist in the apparatus increases and, in the worst case, the mist adheres to an electrical contact part, which results in an inferior operation of the printing apparatus. Even when the liquid droplet size is not so small, in a case when using two printing heads of dark and light by combining the above-stated method (1) and method (2), the ejected satellite amount likewise increases because the number of heads is increased. Thus, the same problem occurs.